I know that, but I got the same skin as the char tier 3 shield from a heart vendor in (not sure) Iron marches. So I'm guessing the tier 3 Asura weapons have something similiar, that way I don't have to spend 63K karma when the skin can be obtained for much less.

Not all the cultural weapons are available that way. For instance Asuran and Sylvari tier 3 weapons can only be found from their main cities' vendors, possibly due to them having unique animations and the crystal-like parts of the Sylvari items.